This invention relates to inhalers and nasal sprays for medicaments or the like and more particularly to propellant driven inhalers and nasal spray valves.
Nasal sprays and inhalers are designed to introduce medicament into the bodies of users and the distinctions between the two types of apparatus depends on whether the medicament is introduced through the mouth or the nose of the user. However, the technology is similar in both cases and references to inhalers include similar, nasal spray technology.
Inhalers typically consist of a storage container and a metering valve assembly. In the valve assembly a metering volume is provided between two valves. These valves are often on a single stem and act together to allow the medicament to be dispensed from the storage container, via the metering volume.
EP-A-0692434 discloses an aerosol metering valve that is designed to address the problem of drain back. This occurs when a dose of medicament is left in the metering chamber over a protracted period and the medicament seeps through the seal and back into the storage container. An additional seal is introduced to close the path from the metering chamber back into the main reservoir during a long dormant period.
U.S. Pat. No. 3,052,382 and U.S. Pat. No. 3,142,420 disclose a metering dispenser for aerosol with fluid pressure operated piston. A cylindrical metering chamber is provided that is sealed at either end with an elastic sealing washer.
The sequence of operation of the valve assembly generally employed in inhalers today is as follows. When the device is not in use a dose of medicament is stored in the metering volume and the outermost of the two valves, which is generally a face seal, is maintained closed. The inner valve is open at this time allowing fluid communication between the metering volume and the contents of the storage container. When a dose is to be dispensed, the inner valve is first closed immediately prior to the outer valve opening. In this way when the contents of the metering volume are discharged, the escape of the medicament stored in the container is prevented. Dispensing is generally achieved by the container being depressed within a housing, either directly by the action of the user or by means of an operating mechanism. When the medicament has been dispensed, the valve sequence reverses as the container is allowed to return to its original position and state: thus the outer valve closes prior to the inner valve opening and the metering volume refills with medicament from the container. The inhaler is left in this state until it is required again for use.
Such a dispensing cycle is a necessary feature particularly of manually operated conventional devices, since the dose retained in the metering volume during the storage has to be dispensed immediately on demand.
Breath actuated devices currently on the market retain this valve technology, but in general operate by loading a spring prior to inhalation by means of some user action such as opening a mouthpiece cover. This stored energy is then released automatically, to depress the pressurised container within the housing, as the patient inhales.
The advantage of breath actuated inhalers is that they eliminate the need to coordinate the press and breathe actions of manually actuated inhalers. As a result of this the deposition levels of the drug in the lung are not dependent on the user's coordination skills. However, the high force required to fire the canister gives rise to production design compromises. Firstly, the size of the device: to fire the canister typically requires a large spring to be fitted into the design; this can be problematic as users prefer small discrete devices. Furthermore, the large firing force must be released by the small force available from the patient's inhalation. To achieve this requires a mechanism that gives a high gearing of the breath force, typically 500:1. This generally has to be achieved by a multi-stage mechanism with several components resulting in a relatively complex mechanism.
A breath operated inhaler that uses the present canister valve mechanism could be of much simpler design because it is not necessary to have a source of stored energy, for example, a large spring, to release the dose.
Generally there is a desire to make inhalers as small and discrete as possible. The necessity for a high force spring in the device means that small, thin walled components must endure high stress levels. This often requires more costly, high performance polymers to be used. The problem is particularly acute in devices with dose counting. Such devices typically require that an electrical switch is actuated, or mechanical counting mechanism is indexed at the point in the stroke where the canister releases its dose. To avoid the danger of counting errors the counting and dose release should ideally occur at the same point in the canister stroke. For highly stressed materials, displacement due to creep can mean that the point at which dose counting occurs drifts over the life of the device making counting errors more likely.
The manufacture of inhalers presently on the market generally involves assembling the inhaler and then to filling it by forcing the medicament and propellant, either simultaneously or separately, back through the valves and into the canister. Two problems arise from this prior construction and method of manufacture. The first is that the valves can be damaged by the considerable pressure required to force them open to fill the canister. Also, there is no provision for varying the volume dispensed except by varying the concentration of the medicament supplied to the canister.
In conventional canister valves a dose is delivered into the metering chamber immediately after the previous dose is released. The canister is stored with the metering chamber full and the dose must be contained therein for the time between taking one dose and the next. Inhalers, in general, whether manually or breath operated, suffer from the problem that the dose in the metering chamber can diminish over time either by escaping through the seals of the outer valve or by drain back into the storage container if the canister is stored inverted. If the metered dose in the metering chamber leaks away, it will not refill even if the canister is subsequently stored upright. The reduction in the dose in the metering chamber then causes the user to receive a lower than expected dose at next usage.